The following publications are representative of the most pertinent prior art known to the Applicant at the time of the filing of this application.
______________________________________ U.S. PAT. NOS. ______________________________________ 3,342,616 Alper et al Sept. 19, 1967 3,492,383 Heimgartner June 27, 1970 3,587,198 Hensel June 28, 1971 3,625,721 Snyder Dec. 7, 1971 3,751,571 Burrows Aug. 7, 1973 3,767,375 Brichard et al Oct. 23, 1973 3,974,249 Roy et al Aug. 10, 1976 4,138,455 Shaikh et al Feb. 6, 1979 ______________________________________
A containment problem has arisen in the manufacture of certain alloys of aluminum and particularly alloys of aluminum and lithium. The metals are melted together in either channel or coreless induction furnaces and the molten metals are transported or carried or passed through various processing steps in ladles, filter boxes, tundishes or come in contact with mold tops etc. These elements of the metal processing equipment are lined in various ways with refractory elements designed to hold the molten metal and protect the structural support elements of the equipment from chemical attack and the thermal stresses produced by the high fusion temperature of the fluid bath contained in srch equipment during the melting and alloying of the metals.
Various kinds of refractory liners have been proposed for use in induction furnaces and the like as illustrated in the following United States Patents:
U.S. Pat. Nos. 3,492,383 and 3,587,198 are early examples of plural layers of refractory coatings in furnaces. The earlier patent shows a sintered liner that is backed-up by a cushion layer designed to contain molten metal in an induction furnace.
The Burrows U.S. Pat. No. 3,751,571 shows a somewhat more advanced version of the cushion layer design. In Burrows, a monolithic cast and cured liner is formed in a furnace, the inner surface of which is sintered when a melt is produced in the furnace to produce a crust backed-up by a soft friable zone that has a hard hydraulically cured outer zone beyond the friable layer. This tri-zone type of liner tends to protect the supporting structure from heat damage and corrosive metal attack by providing the friable layer to catch molten metal that issues through cracks that inevitably form in the sintered inner face.
U.S. Pat. No. 3,767,375 discloses the use of prefabricated refractory bricks having the joints between the bricks filled with a powdered material such as carbon, alumina, chromia or other material that is not wetted by molten metal so that the metal does not penetrate through the restricted spaces between the particles of the filler material.
As taught herein, a preferred refractory liner for induction furnaces and the like can take the form of a dry cement that may be placed in the vessel to be protected and vibrated into a compacted layer. The dry vibrated layer is then subjected to a sintering action to form a crust or hard layer on the surface of a semi-sintered and/or unsintered compacted cement forming a back up support which retains the form of a compacted plastic or granular layer behind the sintered surface layer to insulate the structural support means of the vessel from direct contact with the molten bath and simultaneously produce a stress accommodating layer between the sintered crust and back wall or support for the molten liquid container. Such refractory liner is well adapted to provide a vessel for holding the molten metal while protecting the supporting structure from thermal expansion without subjecting any part of the furnacing equipment to an undue stress.
This above described invention builds on the above noted patented teachings and provides a container structure for particular kinds of molten alloys wherein a sintered crust is backed-up by a semi-sintered and/or granular layer to control the transmission of heat and expansion stresses from the container to the support means. The granular backing layer serves a further function as here taught by having included therein materials for reacting with any spurs of molten alloy metal flowing through the inevitable cracks that form in the sintered container to form reaction products that seal off the crack and prevent the further flow of molten metal much beyond the containment vessel. This desired result is accomplished by providing a dry vibratable cement composition having a specific chemical composition adapted to react with the aluminum lithium, or aluminum magnesium or aluminum zinc alloy being produced, as will appear more fully below.
The novel cement proposed for such use herein includes a base grain component of stoichiometric, or magnesia rich hyperstoichiometric spinel or magnesia grain either calcined or fused mixed with the added flux mix also containing a lithium fluoride flux added to this base grain, fine fractions of alumina, silica magnesia and/or spinel fractions.
In connection with the chemical aspects of this invention, it is known that lithia stabilizes periclase based solid refractory solutions including spinel as taught in U.S. Pat. No. 3,342,616. This stabilized refractory composition has been suggested for use in metalurgical furnaces, refractory tubes, spark plug insulators and the like.
A permeable refractory brick has been formed with alumina, a lithium compound, a small amount of magnesia and other metal oxides as shown in U.S. Pat. No.3,625,721.
In a non-related art, a patent has been noted showing a combination of materials like those shown herein, for example U.S. Pat. No. 3,974,249 shows a method of forming a solid light transmitting refractory body that is prcduced by combining equal molar amounts of magnesia and alumina that can be calcined in the presence of a small addition of lithium fluoride to react the mass to form spinel, which can then be cooled and pressed to form a self sustaining compact that can then be fired in a wet hydrogen atmosphere to produce a transparent ceramic body.